This invention relates to heat exchangers and more particularly to the protection of heat exchangers such as heat pipes from damage resulting from freezing of a working fluid within the heat exchanger.
A potential problem with heat exchangers such as heat pipes is structural damage to the heat pipe caused by freezing of its working fluid, particularly when water is employed as the working fluid. In appliances using a heat pipe this can occur in cold weather during storage or transport of the appliance. It can also occur after installation if the appliance is subjected to freezing conditions--e.g., because of power or mechanical failure. Such freezing can lead to the rupture of a wall or joint of the heat pipe and to failure of the heat pipe.
One known method of avoiding damage is to prevent freezing of the working fluid by adding to it sufficient amounts of anti-freeze agents. For example, when water is used as a working fluid, ethylene glycol may be added to depress the freezing point of the working fluid. However, the large amounts of anti-freeze agents required to prevent freezing (e.g., up to about 50 percent by volume to depress the freezing point of water to minus 30.degree. F.) produces a mixture whose characteristics differ substantially from those of the working fluid. This can result in altered performance and reduced heat transfer by the heat pipe. Moreover, anti-freeze agents may be expensive when required in large quantities.
Other approaches permit freezing but rely on special wicks or provide expansion relief within the vessel containing a working fluid. The "special wick" approach is illustrated in U.S. Pat. No. 4,194,559, which describes a heat pipe with a high capillary lift wick capable of holding all of a limited inventory of working fluid in the heat pipe. Expansion relief techniques are disclosed in U.S. Pat. Nos. 4,321,908 and 4,227,512 which suggest the use of compressible tubes within conduits or the headers of solar water heaters. Another patent, U.S. Pat. No. 4,248,295, describes a freezable heat pipe containing a free-standing, porous structure whose length exceeds the depth of liquid in the heat pipe. Drawbacks to these approaches include the design restrictions they require and the undesirably large amount of space occupied by expansion relief structures. Moreover, such structures may be difficult to utilize in heat pipes having complex geometry.
Accordingly, it is an object of the invention to provide an improved method of protecting heat exchangers from damage due to freezing.
It is an object of the invention to provide a heat pipe capable of surviving freezing and thawing cycles.
It is an object of the invention to provide a heat pipe partially filled with a working fluid whose freeze characteristics permit freezing of the fluid without rupture of the heat pipe.
It is an object of the invention to provide a method for modifying the freeze characteristics of the working fluid of a heat pipe to prevent damage due to freezing without degrading performance of the heat pipe.